Cyclic nucleotide, especially cyclic GMP, are important in the visual process and in other functions of the retina and pigment epithelium. Deficiencies in the enzymes of cyclic nucleotide metabolism may also be responsible for diseases related to retinal dysfunction. In a dog model for retinitis pigmentosa it appears that a switch in cyclic GMP phosphodiesterase (PDE) fails to occur during development in retinas of affected animals. This, coupled with low levels of a PDE Protein Activator in affected retinas, appears to result in derangement in cyclic nucleotide metabolism that is characteristic of this hereditary degenerative disease.